This invention relates generally to a method for recovering heavy hydrocarbon materials from an underground, unconsolidated, carbonaceous formation. More particularly, the invention relates to an in-situ method of recovering bitumen from an underground tar sand (or oil sand) formation. The utility of the invention lies in the recovery of a crude hydrocarbonaceous material from an underground formation that is not amenable to methods of bitumen recovery as applied to surface or near-surface tar sand deposits. Broadly, this recovery process is applicable to a variety of solid or semi-solid natural resources which become fluid when heated (e.g., heavy oil such as found in Canada, sulphur, gilsonite, uintaite, etc.) or which will dissolve or slurry in a liquid solvent (e.g., uranium, coal, gypsum, etc.).
The process does not require surface or underground mining, beyond the preparation of a borehole, and can be applied to resources that are too deep for surface mining or which occur in formations that cannot be safely or economically mined by conventional methods. The process is most effective when the matrix of the host rock is semi-consolidated or of low strength, wherein the matrix can be broken down into smaller pieces. The Athabasca tar sands of Canada are used to illustrate the process.
Several tar sand projects in Canada are in, or near, production. Broadly, these projects involve surface mining methods in removing the tar sand for subsequent separation of bitumen from sand in an extraction plant. These operations involve large scale materials handling, bitumen extraction, tailings disposal, and reclamation steps. Further, these operations depend on the stripping of shallow overburden to uncover the tar sand deposits. Typically, these deposits vary in depth from about 100 to about 300 feet below the overburden and are thus designated as "shallow" deposits. On the other hand, tar sand deposits found at depths greater than about 300 feet, such as from about 300 feet to about 2000 feet, require different procedures and technology.
For these "deep" tar sands deposits, some prior extraction schemes disclose conventional underground mining methods, such as block caving and long wall, with some innovations in design of mining methods or equipment. Other proposed processes utilize hydraulic mining and slurry removal from underground workings. But these methods have the disadvantages of (a) the incompetency of tar sand formations, (b) the peculiar characteristics of rock mechanics of the formations above the tar sands, (c) the impracticality of the large volumes of tar sands (100,000-300,000 tons/day) required from underground mining, and (d) the relatively high mining costs of underground mining.
Mine assisted in-situ bitumen or heavy oil recovery techniques have also bee suggested. The underground workings provide the close proximity to the oil bearing formation for subsequent process steps, such as those to be carried out by a series of angled drilling holes from these underground workings in order to conduct thermal stimulation of the reservoir or to establish inter-well communication within the formation.
Some surface in-situ recovery methods are disclosed for either single well or multi-well applications. These methods offer the advantage of leaving the sand in the ground where it belongs and extracting the bitumen only, thereby reducing the large amounts of materials to be handled. Also, some of the environmental problems of surface mining operations, ground stability, and safety are eliminated or reduced.
Generally, the thermal methods for underground recovery have not been successful because considerable amounts of thermal energy and long periods of time are required to heat up the formation to give satisfactory viscosity and mobility of the desired product. In addition, non-homogeneous formations with clay layers or pockets, sand layers or lenses, and shale streaks often introduce a great deal of difficulty in heating the formation and displacing the bitumen uniformly. Low permeability causes problems in establishing fluid communication within the formation. Finally, heat losses to the overburden increase the heat energy requirements even more. Methods utilizing a solvent (or multi-solvents) to improve the permeability by dissolving the bitumen and freeing it from the rock matrix have not proven to be economically feasible. The initial cost of solvents and solvent losses to non-productive zones contribute to the high cost of these methods. Emulsification processes based on the use of hot water or steam, together with an alkaline additive to reduce viscosity, have been unsuccessful because of the limited area of penetration due to poor permeability.
In U.S. Pat. No. 4,114,687, an initial cavity is formed around a screen placed in the borehole, and a gravel pack slurry is then pumped down into the cavity, filling the cavity around the screen. The tar sands around the screen are washed with hot processing liquid, and the separated bitumen, filtered by the gravel pack, is pumped to the surface. In U.S. Pat. No. 4,124,074, a conical-shaped gravel pack is formed in the initial cavity, and a screen is run down through the cone apex to the bottom of the cavity. Then, hot processing liquid is added, and the melted bitumen flows through the gravel and screen to the surface. The present invention needs no such gravel pack or screen.